If a man has had a vasectomy, can DNA be obtained from his semen?

Can a DNA sample be taken from semen from someone who has had a vasectomy (no sperm)?

— Mary Thomas

Your question, taken at face value, is whether or not DNA can be recovered from semen from a vasectomized male. The answer to this question is "yes," but not for the reasons you may think.

While it is well known that sperm (the male reproductive cell) is found in semen, other cellular components are in there as well. "Stratified simple squamous epithelial cells" is a fancy word for "general, all-purpose wet skin cell." To shorten a semester-long histology class into one sentence, the human body produces cells to line and protect some of the moist parts of the body, namely, the mouth, the rectum, the vagina, and the urethral lining of the penis, to name a few. These cells form a protective layer, like shingles on a roof. These cells have nuclei and thus contain DNA, even in males producing absolutely, positively, zero sperm.

But it's rare that a vasectomized male has no sperm whatsoever. Typically, he still has some sperm, just not as much as he did before the operation. You see, a vasectomy doesn't mean the man stops producing sperm, just that the duct that carries the sperm has been either sealed or severed. When a sealing method is used, some sperm still can manage to get mixed with seminal fluid. So, if what you really wanted to know was whether DNA could be recovered from sperm cells from the semen of a vasectomized male, well, the answer is "sometimes, depending on how complete the operation was."

If, however, you wanted to know whether DNA could be recovered from sperm cells from the semen of a vasectomized male whose surgery was absolutely and totally successful, well, the answer is "no, of course not."

Finally, if your question had anything to do with forensic DNA analysis, like "could a DNA sample be recovered from semen from a vasectomized male," the answer is "maybe." Forensic samples are usually not the whole ejaculated volume (samples tend to be small) and may be degraded and/or contaminated. Of course, that applies to any semen sample, whether from a vasectomized male or not.

But to answer what I take to be your gut question: no, a rapist can't expect to escape detection via DNA analysis just because he had a vasectomy.

— Hawk

Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.